1. Field of the Invention
This invention relates generally to pistons and cylinders and more particularly to double-acting pistons for pneumatic actuators.
2. Description of the Prior Art
Piston and cylinder assemblies for various applications such as engines, compressors, pumps, and linear actuators typically have rings disposed in circumferential grooves in the piston wall for purposes of providing a seal between the piston and cylinder wall or for facilitating lubrication and keeping the piston from scraping the cylinder wall. In the case of pistons for double-acting pneumatic actuators, which provide reciprocating linear movement in response to pressurized gas alternately applied to chambers in the cylinder facing opposite ends of the piston, the piston conventionally has been provided with a pair of spaced-apart circumferential grooves and an O-ring of plastic or elastomeric material disposed in each groove. The O-ring in such devices keeps the piston out of contact with the cylinder wall, prevents loss of pressure around the piston, and may comprise a material impregnated with a lubricant to reduce friction.
While use of O-rings in piston grooves has proven effective for double-acting pneumatic actuators, certain disadvantages are presented. O-rings in this application eventually wear out, limiting the operating life of the actuator. In addition, even when the O-rings are lubricated, friction between the rings and cylinder wall, which must be overcome on each stroke, results in a time lag between strokes. Such a lag becomes significant where the actuator is sought to be precisely controlled, as by a computerized system. The presence of rings also increase the minimum operating gas pressure for a particular actuation system.
Provision of a gas-bearing effect between a piston and cylinder walls for certain types of applications is disclosed in the prior art. U.S. Pat. No. 4,644,851 discloses a piston for a linear compressor that has a reservoir inside the piston along with apertures that communicate with gas pockets on the piston wall, thus forming a gas bearing between the piston and cylinder wall. The piston structure shown in this patent is complex, including axial channels on the piston wall surface as well as spaced-apart gas pockets and a recess surrounding the channels and gas pockets. The piston disclosed in this patent is a single-acting piston for compressing refrigeration gases, and its complex structure does not lend itself to economical fabrication for use in linear actuators. Also, no suggestion is made regarding use of such structure for a double-acting piston. U.S. Pat. No. 4,681,326 also discloses a piston that obtains a gas bearing effect by trapping combustion gases between piston rings. This structure, of course, still requires the presence of rings, and the piston is a single-acting piston for an internal combustion engine. Another piston that provides a gas bearing is shown in USSR Author's Certificate No. 454,805. This piston, for a gas compressor, has an internal chamber that communicates with the high-pressure region facing the piston head and with outwardly movable segments disposed in a circumferential groove. Radially extending apertures are also provided for delivery of gas to the space between the piston and cylinder walls. A gas compressor piston having a gas bearing produced by introducing gas into the space between piston and cylinder through holes in the cylinder wall is shown in USSR Author's Certificate No. 883,549, and a similar structure is disclosed in USSR Author's Certificate No. 703,708. The latter two devices require specially fabricated cylinders and other features that would not be feasible for application to double-acting linear actuators. While the above-discussed patents disclose various approaches to achieving a gas-bearing effect between a piston and a cylinder, none of those approaches would be amenable to practical application for linear actuators.